Interstate Numbering System

Eisenhower Interstate System naming rules:

All interstate highways start with the letter I and end in a number.

For north-south interstates, the numbers used are odd. For east-west interstates the numbers are even.

Numbers are assigned by region. For east-west interstates the southern routes use the lower numbers (ex. I-4) and the northern ones use higher numbers (ex. I-92).
For the north-south interstates the lower numbers are in the west and increase toward the east.

All transcontinental (coast-to-coast or border-to-border) interstate numbers end in 0 or 5 (ex. I-65 and I-90).

As a trend, though probably not strictly adhered anymore.

On a larger scale, the middle digit gets larger as you go east, or north. That is, I10 is very southern and I90 is very northern; I5 is very western and I85 is very eastern.

The San Francisco Bay Area has: I80, I280, I380, I480, I580, I680, I880, I980. Most metro areas have at least an I4xx. Dallas and Ft. Worth have a special split: I35E and I35W allow the I35 to visit both cities.

From Chapter 6 of "Read Your Road," a publication of the Federal Highway Administration.

http://safety.fhwa.dot.gov/factsfigs/ryr/chapter6.htm, Accessed July 31, 2000.

Decoding the Interstate Numbering System

When you're traveling in unfamiliar territory, Interstate numbers give you valuable clues to your location and direction, if you know how to read them. Here's the key:

One- or two-digit even- numbered Interstates are always east-west routes. The numbers increase from south (I-10) to north (I-94).

One- or two-digit odd-numbered Interstates are always north-south routes. Numbers increase from the West Coast (I-5) to the East Coast (I-95).

TIP: Since not all Interstates run due east-west or north-south, the closest cardinal direction that applies to most of the road is used. So, even if a stretch of an east-west Interstate shifts to the south for a while, it is still considered an east-west highway.

Three-Digit Interstates

If the first of the three digits is an even number, the highway usually connects to another Interstate at both ends - often in a circular beltway or loop.

If the first of the three digits is an odd number, the highway is usually a "spur" route that connects with an Interstate at only one end, sometimes going into a city center.

Exceptions to the Interstate Naming Rules

I-238

There is no Interstate 38, but there is an Interstate 238. It runs east-west just south of Oakland, California, connecting I-580 in Castro Valley to I-880 in San Leandro.This 2 mile long portion portition of California State Route 238 was upgraded to Interstate highway status in 1983, but there were no available numbers ending in -80 in the San Francisco Bay Area, so the American Association of State Highway and Transportation Officials (AASHTO) granted an exception to the rules.

Interstate H1, Interstate H2, Interstate H3

These "Interstate" highways on the Hawaiian island of Oahu run the wrong ways by the mainland numbering rules - H1 & H3 run east-west, and H2 runs north-south.